How to "impose" texture while sculpting?
To sculpt with a texture, go into sculpt mode, select the draw brush, and add a texture by clicking the New button as shown below:

Then go into the texture panel, and select an image for your texture with the open button:

Then, to be able to draw the texture in place while controlling the size as in the video, you need to set the stroke mode to Anchored in the brush settings:

Now just ensure your mesh has enough geometry (or use dyntopo with a high level of detail), and you can sculpt a texture onto your mesh.